Converting cobbled 7747 TBI setup to standard ~1990 truck

**Mastiff** · 09-10-2022

Hey guys. I've got a 83 Blazer that I converted to 7747 TBI about 20 years ago. A place called CustomEFIs helped me out and provided the distributor shown below. It appears to be a non CC distributor adapted to CC somehow. You can't tell here, but it's also coil-in-cap, where a standard 1990 truck TBI was external coil. I'm starting to have some issues and would like to convert to a standard/"normal" TBI distributor with external coil. These distributors have two exposed connectors from the module at the base with 2-pins and 4-pins.

Looking at what I have here, there are 4 wires going in on the left, which go straight to the module. On the right side there are 3 wires going into the distributor, 2 of them go to the module and another going to ground (I think). Does anyone understand this setup and how the connections should map to a "real" TBI distributor? I'd really prefer not to have to trace every wire back to pins on the ECM if I can avoid it. Thanks.

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**dave w** · 09-10-2022

Back in the day, I used the 1985 Police Caprice 5.7 liter distributor ( 7 pin ignition module / without vacuum advance) for HEI Large Cap TBI conversions. Maybe CustomEFI's used the same distributor?

RockAuto.com information
CARDONE 841865 Includes Cap & Rotor Info One of our most popular parts with Police Package; Electronic Distributor

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**Mastiff** · 09-10-2022

That distributor does look the same, except I wonder if the module is swapped out. From my picture it appears to only have two connections to the actual module inside. The part number on my module matches this one, a 6-pin:

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**stew86MCSS396** · 09-10-2022

Not sure if this is relevant but Early C4s used HEIs. Seem to recall that between small and big cap, there's some spark latency that needs to be adjusted, no?

This thread comes at an opportune time...I need to take the guts out of one of those and transfer it to a Pontiac HEI. I'm sure it's as easy as having both distributors laying next to each other and swapping parts.

**1project2many** · 09-10-2022

The early computerized distributors were very similar to what you have there. The main difference is that you can see the disabled mechanical advance mechanism on the top of the distributor shaft. GM began using a 7 pin HEI module when vehicles were switched to computer control. Luckily GM kept the wire colors the same for many years. For the ECM circuits, tan/black is the bypass circuit. Purple or Purple and white provides reference pulses to the ecm. White carries the spark control signal from the ecm.
Wires on the right include ignition + to power the module, ground, and coil - to trigger the coil.

Later small cap, external coil distributors use an updated module. Wiring is the same but there is an additional pin for a dedicated ground connected to the ecm. This pin is labelled "G" on the module and is connected to a black / red wire in the harness. The ground wire is removed from the coil harness so there is only ign+ and coil -.

http://www.useasydocs.com/details/GM_7pinHEI.htm

http://www.fieros.de/en/v6help/5s19236e.gif

The 7747 wiring diagram on gearhead-efi will also show the 8 pin module.

**Mastiff** · 09-11-2022

Is there any trouble I can get into if I just buy a distributor like the attached and start adapting the harness connections? Like I think the four wires currently going into the distributor could just connect to the 4-pin connector, and the 2-pin connector will go to the coil, etc., as per the diagram posted above.

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**dave w** · 09-11-2022

Either type distributor; 7 pin module ~ large cap with integral coil or 8 pin module ~ small cap with remote coil can be configured to work.

For ECM controlled spark advance, the 7 pin or 8 module is the preferred setup.

Remote mounted coil connectors
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Remote Coil to 8 pin ignition module pigtail

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